This invention relates to electrophotographic reproduction systems and in particular to an electrophotographic process and apparatus in which an image is transferred from a novel dielectric surface to a receiving surface such as plain paper.
Electrophotographic processes are well known with many ramifications having been disclosed since the earliest disclosure thereof by Chester F. Carlson in U.S. Pat. No. 2,297,691. In that patent a photoconductive insulating material such as sulfur, anthracene, anthraquinone or selenium was utilized as the imaging surface. Since then various other types of photoconductors have been disclosed. Inorganic crystalline photoconductive compounds such as zinc oxide, used in an insulating organic resin binder, were disclosed in U.S. Pat. No. 3,121,006 to Arthur E. Middleton and Donald C. Reynolds. Various organic photoconductors have also been described as imaging surfaces. Polyvinylcarbazole is described as a photoconductor in U.S. Pat. No. 3,037,861 to H. Hoegl, O. Sus and W. Neugebauer. Other organic photoconductors which are known for this purpose include the polyvinylbenzocarbazoles disclosed in U.S. Pat. No. 3,751,246 to Helen C. Printy and Evan S. Baltazzi and the polyiodobenzocarbazoles described in U.S. Pat. No. 3,764,316 to Earl E. Dailey, Jerry M. Barton, Ralph L. Minnis and Evan S. Baltazzi, both assigned to the same assignee as this invention.
Other classes of organic photoconductors are the benzofluorenes and dibenzofluorenes described in U.S. Pat. No. 3,614,412 to William J. Hessel and the cumulenes described in U.S. Pat. No. 3,674,473 to Robert G. Blanchette also assigned to the same assignee as this invention.
The organic photoconductors are generally utilized in conjunction with a suitable sensitizer to extend the spectral range of the photoconductor. Dyes may be used for this purpose. Another class of materials which are widely used are the pi-acids. Representative of these compounds are the oxazolone and butenolide derivatives of fluorenone, described in U.S. Pat. No. 3,556,785 to Evan S. Baltazzi, the dicyanomethylene substituted fluorenes, described in U.S. Pat. No. 3,752,668 to Evan S. Baltazzi and the bianthrones described in U.S. Pat. No. 3,615,411 to William J. Hessel, all assigned to the same assignee as this invention.
Prior art electrophotographic processes in general consisted of uniformly charging a photoconductor electrostatically in the dark, exposing the uniformly charged photoconductor to a pattern of light and shadow corresponding to the image to be reproduced, whereby the photoconductor was selectively discharged in the light-struck areas, contacting the resulting latent electrostatic image with a contrasting colored dielectric powdered material known as a toner to develop the image and either permanently fixing the toned image to the photoconductive surface or transferring it to a suitable receiving material such as a sheet of paper and thereafter permanently fixing the visible image to such receiving material.
Instead of uniformly charging a photoconductive surface with an electrostatic charge, use has been made of a conductive grid or screen upon which is deposited a layer of photoconductor which itself is photoconductive and acts as an ion modulating device. The photoconductive screen is first uniformly charged and then exposed to the image to be reproduced so as to produce a latent electrostatic image thereupon. A stream of ions impinged upon the screen is modulated by the latent electrostatic image thereon so that a latent electrostatic image comparable to that on the screen is formed upon a dielectric imaging surface placed in the path of the ion stream. The resulting latent electrostatic image is then developed upon the imaging surface, transferred to a sheet of paper or other receiving material and then permanently fixed thereto. The use of such screens is described, for example, in U.S. Pat. No. 3,220,324 to Christopher Snelling.
The various prior art processes disclosed above have certain disadvantages. For example, where it is necessary to transfer a toned image from a photoconductive surface to a receiving material, such as a sheet of plain paper, it is necessary to use a corona discharge in order to accomplish the transfer. The primary disadvantage of using corona transfer is that the corona discharge destroys the usefulness of the latent image which must therefore be reformed for each copy. In addition in such processes the transferred image must be fixed following transfer, usually by means of heat or pressure. These shortcomings add process steps which put an upper limit on copying speed. Where the image is fixed upon the photoconductive surface itself, it is also necessary to use as the photoconductive surface a costly, specially formulated material such as zinc oxide coated paper rather than plain paper.